Key takeaways
- The four common house foundations in the UK are strip, trench fill, raft and pile, and each suits different ground conditions.
- Ground conditions, not preference, decide the right foundation: clay, trees, made-up ground and sloping sites all change the design.
- Building control inspects foundations before any concrete is poured, so the trench must be signed off at the right depth first.
- Getting foundations right protects the whole structure above, which is why extensions and new builds both start underground.
The main types of house foundations used in the UK are strip, trench fill, raft and pile. Which one you need depends on the ground beneath your plot, the load of the building above, and nearby factors such as trees or a sloping site.
Foundations are the part of a build nobody sees and everybody relies on. Choose the wrong type, or build it too shallow, and the problems show up later as cracks, sticking doors or movement in the structure. This guide walks through the four common types of house foundations, explains how your ground decides the right one, and shows why foundations matter just as much for a single-storey extension as they do for a full new build.
The Four Common Types of House Foundations
Most domestic projects in and around Northampton use one of four foundation types. Each transfers the weight of the building safely into the ground, but they do it in different ways.
- Strip foundations. A continuous strip of concrete running under load-bearing walls. The wall is then built up from the strip using footings. Strip foundations suit firm, stable ground at a reasonable depth and are the traditional choice for many houses and extensions.
- Trench fill foundations. A deeper trench filled almost to the top with concrete, so the wall starts near ground level. Trench fill uses more concrete than a strip but less labour, and it works well when you need to dig deeper to reach stable ground, for example on shrinkable clay.
- Raft foundations. A single reinforced concrete slab that spreads the load of the whole building across its full footprint. Rafts suit weaker or variable ground where you want to avoid lots of deep digging, and they are common where soil conditions are poor.
- Pile foundations. Columns driven or bored deep into the ground to carry the load down to a stronger layer below. Piles are used where the surface ground cannot support the building, near trees, or on made-up and filled land.
In practice the choice comes down to what is under your feet. A builder does not pick a foundation from a catalogue, they match it to the ground.
How Ground Conditions Determine the Right Foundation
Ground conditions are the single biggest factor in choosing between the types of house foundations. Before any design is finalised, the ground is assessed through trial holes and, on larger jobs, a soil investigation. What that reveals changes everything about the foundation.
Firm gravels and dense sands are forgiving and often allow shallower strip foundations. Soft clays, peat and made-up ground are far less stable and usually call for trench fill, a raft, or piles to reach something solid. The UK's Approved Document A and the wider Building Regulations set out how foundation depth and design respond to soil type, and building control will expect your foundations to follow that guidance alongside the NHBC Standards.
Shrinkable clay deserves special mention because it is common across the Midlands. Clay swells when wet and shrinks when dry, and that seasonal movement is a leading cause of subsidence.
UK subsidence claims reached £153 million in the first half of 2025, supporting almost 9,000 households at an average payout of £17,264 per claim, according to the Association of British Insurers. The ABI noted that climate change is increasing subsidence risk in areas with clay-rich soils.
Deeper foundations on clay get below the zone where the soil dries out and moves, which is why trench fill is so common on these sites.
Why Foundations Matter for Extensions and New Builds
Foundations carry the entire weight of the structure above and hold it steady over decades. On a new build, the whole house depends on them. On an extension, the new foundations also have to sit alongside your existing ones without pulling the two structures apart as they settle at slightly different rates.
This is why extension foundations are rarely a simple copy of what is already there. The depth of your current foundations, the ground conditions and the position of any drains all feed into the design. Foundations are the first stage of the wider groundworks package, and if you want the bigger picture, our guide to what groundworks involve explains how they fit together with drainage, slabs and services.
Getting this stage right protects everything built on top. Rushed or under-designed foundations are expensive to put right once the walls are up, so time spent underground at the start saves money and stress later.
Dealing With Trees, Clay Soil and Sloping Sites
Some sites bring extra challenges that push the foundation design well beyond the standard. Three of the most common are trees, clay soil and slopes.
Trees near the building
Tree roots draw moisture out of the ground, and on shrinkable clay that drying causes the soil to shift. Build too close to a tree, or on the site of one recently removed, and the ground can heave or settle for years. Foundations near trees are usually deeper, and the species, mature height and distance of the tree all affect how deep they need to go. NHBC Standards include detailed tables for this, and deep foundations close to trees are often designed by an engineer.
Clay soil
As covered above, clay moves with the seasons. On clay sites, foundations are taken deeper to reach ground that stays stable regardless of the weather. Trench fill is the usual answer because it reaches depth efficiently and resists the sideways pressure that clay can exert.
Sloping sites
A slope means the foundation cannot sit at a single level. Instead it is stepped down the gradient in stages, keeping each section at the correct depth into firm ground. Stepped foundations need careful setting out so the steps are sized correctly and the structure above stays properly supported.
On any of these sites, the extra work at foundation stage is what keeps the building safe for its whole life, and skipping it is never worth the risk.
Building Control Inspections for Foundations
Foundations are a notifiable stage under the Building Regulations, which means building control must inspect them during construction. The key inspection happens after the trench is dug and before any concrete goes in. An inspector checks the depth, the width, the bearing of the ground at the bottom of the trench and the position relative to drains and boundaries.
If the ground at the base of the trench is not what the design assumed, for example softer than expected or affected by roots, the inspector can require a deeper dig or a revised design before work continues. Only once the trench is signed off should concrete be poured. Keeping photographs of the open trench and its depth is good practice and helps if any questions come up later.
Booking inspections at the right time keeps a project moving. A missed foundation inspection can mean opening up completed work to prove it was built correctly, which is exactly the kind of delay a well-run build avoids.
How AAS Group Gets Foundations Right First Time
At AAS Group we treat foundations as the stage that sets the tone for the whole project. We assess the ground before we design, dig trial holes where needed, and match the foundation type to what we actually find rather than what we hoped to find. That means strip where the ground allows it, trench fill on deeper clay, and rafts or piles where conditions demand them.
We coordinate building control inspections so the trench is checked and approved before concrete is poured, and we handle the complications that catch people out, from trees and clay to stepped foundations on sloping plots. For homeowners and developers across Northampton, that groundwork is the difference between a build that lasts and one that causes problems down the line. #Construction #Northampton
If you are planning an extension or a new build and want foundations done properly from the ground up, get in touch with AAS Group for a straightforward assessment of your site.
Frequently Asked Questions
What is the most common type of house foundation in the UK?
Strip and trench fill foundations are the most common for houses and extensions. Strip foundations suit firm ground, while trench fill is widely used where the trench needs to go deeper, such as on shrinkable clay soils common across the Midlands.
How deep do house foundations need to be?
It depends entirely on the ground. Foundations must reach stable soil below the zone affected by frost, moisture change and tree roots. On shrinkable clay or near trees they go considerably deeper, and your design must follow Approved Document A and NHBC guidance, confirmed by building control.
Do I need building control approval for foundations?
Yes. Foundations are a notifiable stage, so building control inspects the open trench before any concrete is poured. The inspector checks depth, width and the ground conditions at the base of the trench, and work should only continue once that stage is approved.
Which foundation is best for clay soil?
Trench fill is usually the preferred choice on clay because it reaches the required depth efficiently and resists the sideways pressure clay can create. The exact depth depends on how shrinkable the clay is and whether trees are nearby, which is why ground assessment comes first.
